Pressure sensors having a hollow cylindrical metal stem and a sensing element that outputs an electrical signal in proportion to detected pressure are generally known. Such a pressure sensor is disclosed, for example, in Japanese Patent Laid-Open Publication No. 2001-272297.
In the pressure sensor shown in this publication, a flexible diaphragm formed at one axial end of the stem is deformed by pressure transmitted into the hollow part of the stem through an opening that is provided at the other axial end of the stem.
A sensor chip provided on the diaphragm outputs an electrical signal in proportion to the deformation of the diaphragm.
The sensor chip is electrically connected to a wiring board, which is arranged outside the outer circumference of the stem or beyond the perimeter of the stem. The sensor chip is thus electrically connected to an external circuit through the wiring board so that the signal from the sensor chip is output thereto or to another external device.
In this sensor, the wiring board includes a circuit chip for processing the electrical signal from the sensor chip. The signal from the sensor chip is, for example, amplified or converted into a signal that is easier to handle by a processing circuit in this circuit chip before being output.
This type of pressure sensor is usually used for brakes. The sensor was conventionally mounted on an actuator for size reduction of the actuator. Now there is a demand to integrate the pressure sensor with an electromagnetic valve that is also mounted on the actuator in order to reduce the size of the actuator for further saving of space.
When integrating such a pressure sensor with an electromagnetic valve, usually, the valve is axially connected to the stem of the pressure sensor. The electromagnetic valve generally includes a pressure path inside for a pressurized medium to flow in, and a magnetic-driven valve, such as a solenoid, for controlling the flow rate of the pressurized medium in the path.
The pressure is thus controlled by the electromagnetic valve and applied to the diaphragm at one axial end of the stem of the pressure sensor through the opening at the other end. The sensor chip outputs an electrical signal in proportion to the pressure-induced deformation of the diaphragm, whereby pressure sensing is achieved.
Usually, the pressure sensor is inserted in a tubular solenoid, and the electromagnetic valve is driven by magnetic force from the solenoid for the pressure sensing.
Therefore, there is a need to reduce the diameter of the pressure sensor, i.e., of the stem in the sensor. To integrate the sensor with the electromagnetic valve, the sensor diameter (or the stem diameter) must be reduced to at least about 7.5 mm.
However, the radial dimension of the conventional pressure sensor disclosed in the above-identified publication is relatively large because it includes the area for the wiring board that is located outside the perimeter of the stem.
Furthermore, because of a circuit chip that is usually mounted on the wiring board as with this conventional sensor, the size of the wiring board is accordingly large as it includes the mounting area for the circuit chip.